Cruelty towards those who are relatively weaker or indeed defenceless - children, the elderly, partners, animals.

Abusing service / customer-facing staff fits in to this as well and is at once particularly revealing and particularly damning. Next time you’re out and about with friends or a love interest watch how they treat (or talk about) e.g. the person at a ticket booth or the person waiting on tables - if they’re nasty to them (or about them), imagine what they might be like behind closed doors (and how they might treat you one day).